In general, the second and third generation (2G and 3G) mobile communication systems employs a frequency division duplexing (FDD) scheme. In the FDD scheme, a transmission signal and a reception signal are separated by a duplexer. However, recently 3.5G and 4G mobile communication systems are expected to generally use the TDD scheme.
The time division transmission scheme, such as the TDD scheme, discriminates between transmission and reception signals using the same frequency by means of time division, in which bidirectional communication is performed with one frequency by dividing the inside of one frame into transmission and reception sections.
FIG. 1 is a block diagram illustrating the construction of a transmission/reception terminal unit in a conventional TDD system. A transmission signal “Tx” is amplified by a power amplifier 40 to have a predetermined power, passes through a transmission/reception changeover switch 10 and a full-band filter 50, and is then radiated through an antenna 60. In contrast, a reception signal “Rx” received through the antenna 60 passes through the full-band filter 50 and transmission/reception changeover switch 10, and is then amplified through a power amplifier 20 (e.g., low-noise amplifier (LNA)) for reception signals, which has been established to have a proper gain. The changeover switch 10 may perform a switching operation according to switching control signals which are provided by a controller (not shown) based on transmission and reception operations.
Since the TDD system separates transmission and reception according to a predetermined time period using the same frequency, as described above, the TDD system must include an RF switch for high transmission power and high-speed changeover between transmission and reception.
Since the RF switch must enable a high-speed switching operation, a switch (such as a PIN diode or field-effect transistor (FET)) using a semiconductor device, rather than a mechanical switch, is generally used as the RF switch. However, there is a difficulty in using such a switch utilizing a semiconductor device as a high-power switch because of semiconductors' inability to handle high power.
In other words, when high power is applied to the switch, a large amount of heat is generated and may destroy the switch if sufficient heat radiation is not guaranteed. For this reason, an RF switch to withstand high power has been developed. However, the RF switch for high power is very expensive because it must include a separate cooling radiator and so on, and the manufacture thereof is not easy, so that the RF switch is restrictedly used only for military purposes.
In order to solve such a problem, a conventional TDD system employs a method of fixedly separating transmission and reception signals by means of a circulator 10, as shown in FIGS. 8 and 9, instead of an RF switch. However, when such a circulator 10 is used as a switch, there is a difficulty in securing an isolation enough to intercept a transmission signal during a reception section. Also, when there is a problem in the antenna during transmission of a transmission power, an antenna may be open, a VSWR values may be degraded, or the supply of the operation power may be interrupted. Then, a transmission signal flows in a receiver, thereby causing system failure or serious trouble in system equipment, so that the quality of radio waves is deteriorated.